Motor-operated valves, which are opened and closed by a driving force of a motor, have prominent features that a large driving force is available and that remote control can be adopted to open and close the valves. Thus, motor-operated valves are utilized in a variety of applications, such as large-scale plants in which a large-diameter piping system is installed or nuclear power plants in which a manual operation is not permitted. Motor-operated valves are generally used more to fully open or close a pipe in a piping system than to control the flow rate of a fluid.
Such a motor-operated valve has a valve disc for opening and closing a pipe in a piping system, a worm configured to be rotated by a motor, a valve driving mechanism which receives torque from the worm to drive the valve disc, and a spring cartridge having a plurality of belleville springs which are stacked and packed with a preload and which are compressible in proportion to a thrust from the worm. The torque of the motor is transmitted to the valve driving mechanism via the worm to drive the valve disc. When the amount of compression of the belleville spring pack reaches a limit, the motor is stopped to prevent an excessively large driving force from being exerted on the valve, so that the valve disc is prevented from being damaged.
In order to maintain the function of such a motor-operated valve over a long period of time, it is necessary to maintain the motor, worm, valve driving mechanism, valve disc and so on in good conditions. Examples of point to be checked include: whether the valve disc is driven by an appropriate torque, whether an appropriate retention torque is exerted on the valve disc when it is in the fully open or close position, whether the setting of a torque switch for automatically stopping the motor when a predetermined amount of torque is applied to the valve disc is appropriate, and how much the valve driving mechanism and so on are worn. Among these points, the points relating to torque are the most important in view of the fact that most failures of motor-operated valves are attributed to inappropriate torques. Thus, various methods for diagnosing the conditions of a motor-operated valve have been suggested.
For example, Japanese Patent No. 2,982,090 discloses a torque measuring method in which the displacement of a worm is regarded as being the amount of compression of belleville springs. Thus, a relationship between the displacement of the worm and the compression force exerted from the belleville springs is previously obtained. In measurement, the displacement of the worm is measured with a position sensor, and a compression force actually exerted from the belleville springs is obtained from the displacement using the previously obtained relationship. Then, the torque actually exerted on the valve driving mechanism from the worm is calculated from the compression force for the diagnosis of the conditions of the valve. According to this method, since the belleville springs are compressed regardless of whether the valve disc is shifted toward the open position or close position, a compression force (and corresponding torque) can be obtained by measuring the displacement of the worm. That is, by obtaining the relationship between the displacement of the worm and the torque in advance as basic data, the torque corresponding to a displacement of the worm can be obtained regardless of the operating direction of the valve disc.